1. Field of the Invention
The present invention relates to an image heating apparatus which is used for heating an image on a recording material in an electrophotographic or electrostatic recording image forming apparatus such as a printer, a copier, a facsimile, or a multifunction machine having a plurality of functions thereof.
An image heating apparatus includes, for example, a fixing device that fixes an unfixed image on a recording material, and a gloss increasing device configured to heat an image fixed on a recording material to increase the gloss of the image.
2. Description of the Related Art
An electrophotographic image forming apparatus often uses a heat-roller-type or film-fixing-type fixing device configured to fix an unfixed toner image on a recording material. The heat-roller-type fixing device generally includes a fixing roller as a fixing member and a pressure roller as a pressure member that are pressed against each other and rotated. The fixing roller includes a desired functional layer such as a release layer, or an elastic layer and a release layer on an outer periphery of a metal hollow roller base member.
The hollow roller base member is heated from inside by a heater such as a halogen lamp provided in the body, and power supplied to the heater is controlled so that an outer surface of the fixing roller is maintained at a predetermined fixable temperature. The pressure roller has a rubber layer on a circumstance surface of a bar core, and is elastically pressed against the fixing roller to form a nip portion having a predetermined width between the pressure roller and the fixing roller. Generally, a recording material bearing an unfixed toner image is introduced into the nip portion and held and conveyed, and the unfixed toner image is fixed as a fixed image on a recording material surface by the heat of the fixing roller and the nip pressure.
The film-fixing-type fixing device includes a heater such as a ceramic heater fixedly supported, and a tubular film as a fixing member configured to come into contact with the heater. The fixing device also includes a pressure roller configured to be pressed against the heater via the film to form a nip portion between the pressure roller and the film. Generally, a recording material bearing an unfixed toner image is conveyed in the nip portion, and the unfixed toner image is fixed on the recording material by the heat of the film heated by the heater and the pressure of the nip portion.
In the film-fixing-type fixing device, the film is directly heated by the heater. After the power is turned on, the nip portion reaches a predetermined temperature in a short time, thereby significantly reducing the waiting time after the power is turned on. Heating only a necessary section of the film advantageously reduces power consumption. An on-demand device can be configured using a heater and film material having a low heat capacity.
Consequently, the ceramic heater as a heat source may be electrified and heated to a predetermined fixing temperature only when image forming is performed, thereby providing an advantage of a short waiting time from powering on of the apparatus to a state where image forming can be performed, that is, a quick start property, and a significantly low power consumption during standby.
In the heat-roller-type or the film-fixing-type fixing device as described above, when small-size recording materials having a smaller width than a maximum-size recording material having a maximum width that can be passed in the device are continuously passed and subjected to fixing, an excessive increase in surface temperature on a non-sheet-passing area of the fixing member, that is, a temperature at the non-sheet-passing area increase may occur.
This is because if the small-size recording materials are continuously passed, heat is not removed by the recording materials but is stored in the non-sheet-passing area of the nip portion through which the recording material is not passed. This phenomenon is referred to as a non-sheet-passing-area temperature increase of the fixing device, and an excessive temperature increase at the non-sheet-passing area may cause hot offset, or thermal degradation of components that constitutes the device. More specifically, in order to fix an entire area of a maximum-size recording material, for example, an A4 horizontal sheet (297 mm), the temperature of the fixing member in a width direction needs to be uniform over a width or more of the maximum-size recording material.
However, if small-size recording materials having a smaller width than the maximum-size recording material, for example, A4 vertical sheets (210 mm size) are continuously passed, the temperature of the fixing member excessively increases at the non-sheet-passing area, and there is a difference in temperature of the fixing member between a sheet-passing area and the non-sheet-passing area. Thus, if the maximum-size recording material, or a middle-size recording material (for example, B4 vertical sheet) having a larger width than the small-size recording materials continuously passed is passed, hot offset may occur in a section corresponding to a non-sheet-passing area of the small-size recording material to produce a poor image.
The excessive temperature increase on the non-sheet-passing area of the fixing member that occurs in the continuous passage of the small-size recording materials described above is particularly remarkable in a film-fixing-type fixing device including a fixing member having a reduced heat capacity for energy saving. In order to prevent hot offset caused by the temperature increase at non-sheet-passing area, a conventional fixing device provides a period for cooling the non-sheet-passing area after continuous passage of the small-size recording materials.
Specifically, after continuous passage of the small-size recording materials, a period is provided in which a next print job is not accepted for a predetermined time or until a signal value of a detecting unit, configured to detect the temperature of the fixing member or the pressure member on the non-sheet-passing area, reaches a predetermined value. After substantially uniform temperature distribution is obtained over the entire area in the width direction of the fixing member or the pressure member, the maximum-size recording material or the like is passed. However, it takes several ten seconds to several minutes to obtain substantially uniform temperature distribution over the entire area in the width direction of the fixing member or the pressure member, thereby reducing productivity.
A configuration is also known in which an air supplying fan is provided in a fixing device, and a cooling system is provided, which is configured to supply air to non sheet-passing areas on opposite end portions of a fixing roller and a pressure roller and to cool the non-sheet-passing areas, thereby suppressing temperature increase at a non-sheet-passing area. In Japanese Patent Application Laid-Open No. H04-051179, an air supplying fan placed in a fixing device selectively supplies cooling air to non-sheet-passing areas on opposite end portions. Japanese Patent Application Laid-Open No. 2003-076209 discloses a configuration in which when an air supplying fan supplies cooling air to a non-sheet-passing area, an air supplying area of the air supplying fan can be adjusted according to a width of a recording material to be passed to accommodate recording materials of different sizes.
Japanese Patent Application Laid-Open No. 2010-072399 discloses a configuration in which a temperature detecting member, configured to detect the temperature on a non-sheet-passing area of a fixing member, is provided to adjust the quantity of air of an air supplying fan according to a detection temperature.
However, the conventional fixing devices described above, including the cooling system in which the air supplying fan is provided to supply air to a heating roller and a pressure roller in a non-sheet-passing area, have the problems described below. The cooling capacity of the air supplying fan sometimes does not reach a proper level, depending on the types of recording materials that are passed, which may cause poor fixing or hot offset. FIG. 1A illustrates a schematic configuration of a fixing device including an air supplying fan, and its relationship to the temperature distribution in a ceramic heater section. A recording material 401, a ceramic heater heat generating section 404, and air supplying fans 402 and 403 are provided. The recording material 401 is conveyed around a broken line B passing through a central portion of the ceramic heater 404.
The amount of heat generation of the ceramic heater heat generating section 404 is adjusted based on a result of a temperature detecting unit provided at a point Q on the central portion of the ceramic heater heat generating section 404, and the central portion of the ceramic heater heat generating section 404 is controlled to a desired temperature. A line K and a broken line J each show the temperature distribution of the ceramic heater heat generating section 404 when recording materials having different heat capacities are passed. The broken line J shows the temperature distribution when a recording material having a high heat capacity is passed, and the line K shows the temperature distribution when a recording material having a lower heat capacity, as compared to the broken line J, is passed. As shown, the temperature at non-sheet-passing areas G and H is higher for a recording material having a higher heat capacity.
Heat generated by the ceramic heater heat generating section 404 is transferred to a recording material to be passed. At this time, the amount of heat transferred to a recording material increases with an increase in the heat capacity of the recording material. Specifically, the temperature difference between the recording material sheet-passing area and the non-sheet-passing areas G and H of the ceramic heater heat generating section 404 increases with an increase in the heat capacity of the recording material. The temperature increase at the non-sheet-passing areas G and H increases temperatures on recording material end portions L and M to cause hot offset. In order to prevent hot offset, a method is conceivable for cooling the non-sheet-passing areas G and H by uniformly increasing the quantity of air of the air supplying fan.
However, in this case, passage of a recording material having a low heat capacity reduces the temperature at the recording material end portions L and M to cause poor fixing. A method is conceivable of solving the above problem by providing a member configured to detect the heat capacity of a recording material or a member configured to detect an environmental temperature in the device, and adjusting the quantity of air of an air supplying fan according to a detection result, which increases cost of the device.
A method is conceivable of providing a thermistor configured to detect the temperature on a non-sheet-passing area of a fixing member through which a recording material is not passed, and adjusting the quantity of air of an air supplying fan according to the detection temperature. However, if the quantity of air is determined based on the temperature at the non-sheet-passing area, the temperature may overshoot to cause poor fixing or hot offset when the temperature increasing speed is high.
If the quantity of air is uniformly set in response to the case of a high temperature increasing speed on the end portion of the fixing member, overcooling may cause poor fixing. If the quantity of air is set in response to the case of a low temperature increasing speed on the end portion, an insufficient cooling capacity may damage the fixing member (film).